This invention relates to an ink jet recording sheet, particularly to a large size ink jet recording sheet which has a high water-resistance in a severe environmental condition, especially resistant to outdoor use and using an aqueous ink excellent in ink gloss, smoothness, ink absorption and dryability.
The ink jet recording process has been used in various printer for many applications as it is low in noise and can perform a high speed recording and multi-color recording.
Especially, the image formed by the multi-color ink jet recording process has been noticed as it compares favorably with the usual multi-color printing and it can be manufactured in lower cost than the usual multi-color photoengraving and the color photoprint process in the case of large poster in a small number of copies.
As to the recording sheet used in the multi-color ink jet recording process, the woodfree paper and the coated paper used in the general printing cannot be used in the application such as the poster for outdoor use as it has paper substrate and thus is weak to the humidity change and rain.
Baryta paper used as the substrate of the photoprint substrate contains an wet strength improver, etc. to enhance the water resistance but gelatin and casein, etc. used in the coating layer are low in water-resistance and so it is also unsuitable for outdoor use.
On these points, the synthetic paper is excellent in water resistance but inferior in ink absorption and thus, when it is used as the sheet for ink jet recording, the image is contaminated with residual ink. Especially in the case of multi-color recording, the ink amount is high as ink dots of 2 to 4 colors overlaps on the same spot and thus the inks are mixed before absorbed and floods out unfavorably to make the process impractical.
There bas been proposed the use of a coated sheet prepared by forming on a synthetic film a pigment coating layer which is generally used to make an ink jet recording sheet. For example, U. S. Pat. No.4,460,637 (Japanese Laid-Open Patent Publication No.110287 of 1983) discloses a method in which the uppermost layer is formed with use of pigments having a given particle size to obtain the pore radius distribution curve showing at least two peaks. On the other hande, U. S. Pat. No.4,900,620 (Japanese Laid-Open Patent Publication No.95092 of 1989) discloses the use of a secondary particle size silica having a high specific surface area as a white pigment comprised in the coating layer.
However, those methods are insufficient in both of ink gloss and ink smoothness and only gives a dull image compared to usual multi-color print. Furthermore, 1t provides poor color reproducibility to give necessarily no satisfactory result.
From these circumstance, we, inventors, have investigated on the elimination of the above disadvantages especially by aiming at an ink jet recording sheet prepared by forming a pigment coating layer on a film substrate and have found that, when calcium carbonate having a specific oil absorption and shape is used as the pigment constituting the pigment coating layer, ink gloss and smoothness in printing is effectively improved and especially, when an oil absorbing pigment of a specific oil absorption is used in combination with it, ink absorption and dryability can be easily controlled and especially calcined kaolin accomplishes these improvements in very high balance. Furthermore, when a specific resin is used in combination as the adhesive, it has been found that the water resistant strength and the adhesive strength can be improved with no deterioration of this balancing effect to complete the present invention.